This invention relates generally to signal processing techniques for rotation sensor systems used in navigation and other applications. More particularly this invention relates to signal processing techniques in rotation sensor systems that include Coriolis acceleration sensors for measuring rotations.
Angular rate can be measured with a captured linear accelerometer by mounting it on a vibrating frame and measuring the Coriolis accelerations generated by the angular rate of the frame relative to inertial space. Generally to attain precise angular rate measurement, the frequency response of such accelerometers must be well defined at the vibration frequency of the dithered frame. The scale factor is directly related to the accelerometer closed loop gain. Large errors can be generated from the vibration drive motion coupling into the accelerometer if the measure of this motion is not rejected by accurate control of the phase of the reference in the demodulation of the Coriolis signal.
One technique for eliminating the large errors due to uncertainties in the gain and phase of the accelerometer output is to use precision AC torque feedback to exactly cancel the Coriolis forces developed by the rate, thereby maintaining an absolute null of the proof mass at the dither frequency.
In some feedback control systems the parameter being measured varies the amplitude of a sinusoidal carrier signal. In such systems the frequency of the carrier signal is normally much higher than the desired bandwidth for the parameter being measured. Such amplitude modulated signals may be generated from sensors that measure pressure, acceleration, velocity, angular rate, and the like. For some of these sensors, precise measurement of the parameter is determined by measuring the feedback signal required to maintain a balance in a closed loop configuration.
An application where precise measurement of a modulated signal is important is a vibrating angular rate sensor system that measures the Coriolis acceleration generated by an angular rate input. A constant rate input to such a sensor causes an output signal that is amplitude modulated at the frequency of the driven oscillation of the device. The generated Coriolis acceleration is proportional to the input rate and is 90.degree. out of phase with the driven vibration amplitude. Therefore, the maximum acceleration occurs when the maximum vibration velocity occurs, which is 90.degree. out of phase relative to the maximum amplitude of vibration. In most cases the rate sensor is a built-in acceleration detector or a small accelerometer mounted on the vibrating member. The proof mass of the detector responds to the Coriolis acceleration generated by the rate.
If the acceleration sensor is operated in an open loop configuration, then its frequency response must generally be much higher than the driving frequency if the gain and phase of the output signal are to be well-defined. The absolute value of gain is important for scale factor, and the phase of the signal relative to the driven reference oscillation is important in order to reject any "quadrature" signal, which is a major source of error in rate measurement. This same gain and phase difficulty will also occur in closed-loop acceleration sensing if typical capture loop techniques are used.